Improvements In the Manufacture of Semiconductor Devices

摘要

Two IGFETs (Q1, 02) having different threshold voltages due to opposite doping of their gates are produced by depositing a layer 6,6' of undoped polysilicon over the gate oxide 3, using a mask 7 to selectively dope part 6' of this layer, patterning to form doped and undoped gate electrodes 6', 6 and diffusing-in the opposite type impurity to form the source and drain regions 4, 5 and to dope the second gate electrode 6. The conductivity type of the first gate 6' is unchanged either because it is heavily doped or by using a thermal oxidation step to form an oxide layer which grows thicker on the heavily doped gate region than on the undoped gate and lightly doped substrate to produce a masking effect. The devices may be used to provide a voltage reference.